The invention relates to a motor assisted steering apparatus which may be used in a four wheel steering (4WS) apparatus of an automobile, for example, and in particular, to a remedy in the event of a failure thereof.
Various arrangements for a four wheel steering apparatus for automobiles have been proposed and are in practical use to improve the running stability over the steering of the front wheels along and to reduce the minimum turning radius, by auxiliary turning the rear wheels simultaneously with the front wheels during a steering operation. In the four wheel steering apparatus of the kind described, a rear wheel steering drive mechanism which changes the orientation of the rear wheels may be mechanically, hydraulically or electrically driven. Of these, the electrically driven type is advantageous in saving a fuel cost.
A typical prior art is disclosed in Japanese Laid-Open Patent Application No. 89,171/1986, for example. However, when a rear wheel steering drive mechanism which is electrically driven is used, in the even a failure occurs in an electric motor and cannot be driven during the time when the rear wheels are being steered, the rear wheels will be fixed as steered. In such a situation, it is very difficult to drive an automobile inasmuch as the automobile must run continually while it is steered laterally to either side, which prevents a normal driving mode.
To accommodate for such inconvenience, a centering spring may be provided for automatically returning the orientation of the rear wheels to the center in the event of the failure of the electric motor, as disclosed in Japanese Laid-Open Patent Application No 202,977/1986. The provision of a steering wheel which allows the rear wheels to be returned to the central or neutral orientation by a manual operation is also proposed.
Safety is assured by the provision of a centering spring as disclosed in Japanese Laid-Open Patent Application No. 202,977/1986 which automatically returns the rear wheels to the central orientations in the event of a failure of an electric motor or the like. However, it is recognized that the centering spring always urges a steering mechanism with a force which tends to return the rear wheels to the central orientations, and accordingly, a force of an increased magnitude, which is sufficient to cause a flexure of the centering spring, must be used over an arrangement having no such centering spring in order to enable a steering operation even during a normal operation when the rear wheels are to be steered, thus resulting in an increased power loss and requiring an electric motor of a higher rating, while accompanying a degraded response of the rear wheel steering. At the current practice, various mechanical elements are disposed around the rear wheel steering mechanism, which makes it difficult to secure a space for the provision of a steering wheel and for a manual maneuvering thereof.